The development of a quantitative, predictive understanding of solar windmagnetospheric

Transcription

1 White Paper: The development of a quantitative, predictive understanding of solar windmagnetospheric coupling Authors: P. A. Cassak, West Virginia University J. E. Borovsky, Los Alamos National Laboratory M. A. Shay, University of Delaware Relevant panel themes: Solar Wind-Magnetosphere Interactions (SWM) Overview Recent developments in the study of solar wind-magnetospheric coupling have raised questions about traditional views of the coupling process. Understanding how energy from the solar wind gets coupled and transferred into the magnetosphere is a key component of space weather research, as it has implications for how and when geomagnetic storm and substorm activity can arise, the global magnetospheric convection process and how the radiation belts evolve, how aurora are driven, how energetic particles are produced, etc. Therefore, the development of solar wind-magnetospheric coupling models with increasing accuracy is critical for developing a quantitative and predictive understanding of space weather processes. While early work on the subject used simple physical models to explain the coupling, efforts in recent times have furthered a quantitative link through the use of empirical modeling. The advantage of empirical modeling it that it is relatively straightforward; the drawback is that the connection to the underlying physics is lost, which hampers the ability to increase the accuracy of the models and to determine how changes in solar wind parameters impact the subsequent coupling to the magnetosphere. Recent developments by Borovsky (2008) have reawakened interest in the development of a first principles, physics based model of solar wind-magnetospheric coupling. The key departure of this new work relative to previous work is that it is argued that the coupling is proportional to the efficiency of the magnetic reconnection process that occurs when the interplanetary magnetic field meets the terrestrial magnetic field. Previous work assumed that the convective electric field in the solar wind (or some function of it) was the sole driver of the coupling. Borovsky s analysis, even in preliminary form, is finding as good a predictive capability as the best previously obtained empirical modeling.

2 These developments lead to a clear conclusion - the time is ripe for a multipronged and interdisciplinary revisitation to the problem of solar wind-magnetospheric coupling with the goal of resolving what controls solar wind-magnetospheric coupling and how one can develop quantitative predictions which will be of use to space weather applications. In particular, advancing our understanding will require efforts in observational and theoretical/numerical space physics. The purpose of this white paper is to recommend renewed systemic support for research on solar wind-magnetospheric coupling. The impact of sustained decade-long inquiry into this topic will be an enhancement in our fundamental understanding of space physics and an increased ability to predict geomagnetic activity for applications to space weather phenomena. This has been a long term goal of space research, as summarized in The Sun to the Earth -- and Beyond: A Decadal Research Strategy in Solar and Space Physics, the 2003 decadal survey from the National Research Council. Extending and expanding this goal for the next decade is justified because of the exciting new developments in the field. Recent Developments in Solar Wind-Magnetospheric Coupling Observations It has long been considered the case that the efficiency of solar windmagnetospheric coupling is a function only of parameters in the upstream solar wind. In particular, the Akasofu epsilon coupling function (Perrault and Akasofu, 1978) relates the coupling to the convective electric field in the solar wind. The correlation with observational data was good but had much room for improvement [see Newell et al., (2007) for a review]. Since then, the modifications to the epsilon function have been suggested and tested in an effort to improve the correlation with observations. More recently, sophisticated multi-parameter fitting techniques have been used to achieve high levels of correlation between data and assumed functional forms of solar wind parameters. Such efforts have led to vast improvements in the obtainable correlation, nearing 75%. Recent observations, however, provided motivation to rethink this paradigm. When the convective electric field in the solar wind increases abruptly, the enhanced coupling at the dayside magnetopause induces an upflow of plasma from the lower altitudes around the Earth. These so-called plasmaspheric drainage plumes (Borovsky and Steinberg, 2006) consist of relatively cold and dense plasma. When this plasma reaches the site of reconnection, the efficiency of solar wind-magnetospheric coupling is observed to decrease (Borovsky and Denton, 2006). This provides a very clear indication that the parameters in the solar wind are not solely responsible for the coupling. Therefore, complete models must take into account the parameters locally at the magnetopause, specifically those at the reconnection sites where the loading of solar wind energy to the magnetosphere is allowed to take place.

3 Recent Developments in Reconnection Theory Spurred by the observations, interest in magnetic reconnection in magnetopause configurations substantially increased. The main deficiency in the understanding of reconnection in these settings was that the canonical models of magnetic reconnection by Sweet and Parker and by Petschekassume identical plasmas on either side of the dissipation region where the magnetic topology changes, but reconnection at the magnetopause is asymmetric, meaning that the magnetic field strengths and plasma densities are different on t h e m a g n e t o s h e a t h a n d magnetospheric sides (e.g., Phan Schematic diagram of the dissipation region during and Paschmann, 1996). See the asymmetric reconnection, reprinted from Cassak figure to the right as an example. and Shay (2007). There had been many studies on the effect of the asymmetry on the downstream shock structure, but it was not known how the efficiency of reconnection (the reconnection rate) changes due to the asymmetry. Borovsky and Hesse (2007) performed numerical simulations of reconnection with different densities on either side to determine how the reconnection rate changed, finding that the main effect was that the effective Alfven speed of the process is based on a combination of the densities on either side. A first principles scaling analysis, which like the original Sweet-Parker analysis is based on conservation laws, was derived while including asymmetries in both magnetic field and density (Cassak and Shay, 2007). The derived expression for the reconnection rate was consistent with the findings of Borovsky and Hesse (2007). Since, it has been tested further and verified with two-dimensional simulations in resistive magnetohydrodynamics (Cassak and Shay, 2007), magnetohydrodynamics with localized resistivity (Birn et al., 2008), Hall magnetohydrodynamics (Cassak and Shay, 2008), and particle-in-cell (Malakit et al., 2010). Good agreement was also found in three-dimensional global magnetospheric simulations using magnetohydrodynamics (Borovsky et al., 2008). Recent observations of reconnection at the subsolar magnetopause with the Polar satellite are shown to agree well with the predicted scaling (Mozer and Hull, 2010). These studies have also uncovered other new aspects of reconnection. As a result of the upstream asymmetries, the dissipation region develops a different substructure than in symmetric reconnection. The X-line and the stagnation point are co-located in symmetric reconnection, but are separated in asymmetric reconnection (Cassak and Shay,

4 2007). This separation is borne out in the fluid simulations discussed above, as well as in more realistic particle-in-cell simulations (Pritchett, 2008; Pritchett and Mozer, 2009), though details of the model are remain under scrutiny (Cassak and Shay, 2009; Birn et al., 2010; Malakit et al., 2010). Recent particle-in-cell simulations have been profitably applied to direct observations of reconnection at the magnetopause, showing many aspects that are seen in the observations (Tanaka et al., 2008; Mozer et al., 2008a; Mozer et al., 2008b). Also, recent studies employed the results and the analytical technique in studying magnetic reconnection in turbulent plasmas (Servidio et al., 2009) and reconnection with asymmetries in the outflow direction (Murphy et al., 2010). Recent Developments in Solar Wind-Magnetospheric Coupling Functions The development of a first principles prediction for the reconnection rate allowed Borovsky (2008) to develop a coupling parameter for solar wind-magnetospheric coupling which was predicated on local reconnection physics determining the efficiency rather than solely the properties of the solar wind. The so-called Borovsky coupling function was derived using pressure balance arguments to relate local reconnection site parameters to those in upstream in the solar wind. The prediction was compared with observational data, and it was concluded that the correlation was nearly 75%, as is shown in the plot to the right. Thus, the first-generation attempt to incorporate local reconnection physics provided as good a correlation with the data as the best previously Time lagged auroral-electrojet index AE as a function of the Borovsky coupling function in the black dots; a running average of the black points is plotted in red. Reprinted from Borovsky (2008). obtained coupling functions which were obtained using unphysical empirical fitting techniques. Since then, independent studies have tested the coupling function, finding good agreement as well (Turner et al., 2008). The positive results are encouraging, but are clearly not the end of the story. The asymmetric reconnection theory on which the coupling function is based is purely a twodimensional theory (as with the original Sweet-Parker model), but reconnection at the dayside magnetopause can be manifestly three-dimensional (Dorelli, 2008). Borovsky (2008) made assumptions about how the expression could be generalized, but these

5 assumptions would benefit from further verification. Indeed, a recent study (Ouellette et al., 2010) tested the two-dimensional reconnection theory against global magnetospheric simulations, finding good but not great agreement, though they also found that the agreement improved as the resolution was increased. Further, in global simulations with a southward directed interplanetary magnetic field, it was shown that for large enough interplanetary magnetic field strengths, the geoeffectiveness is not proportional to the field because the field convects around the magnetosphere instead of being reconnected (Lopez et al., 2010), an effect which was not incorporated into the model. Proposed Action Due to the recent developments about the nature of solar wind-magnetospheric coupling and the prospective ability to develop increasingly accurate predictions, the coming decade provides a perfect opportunity for a careful and sustained effort on this topic. This should involve a multifaceted approach. Theory and simulations of the fundamental physics of magnetic reconnection - 1) Improved theoretical understanding of how reconnection works at the magnetopause - there remain open questions about the properties of asymmetric reconnection, especially the substructure of the diffusion region and the impact of the substructure on observations. What other aspects play an important role? In many locations on the dayside magnetopause, the convection of the solar wind introduces a shear flow - how does this alter the efficiency of reconnection? How prevalent are other ion species, especially for reconnection near the cusps? Does this alter the reconnection? These studies can be carried out using two-dimensional magnetohydrodynamic, Hallmagnetohydrodynamic, hybrid, and particle-in-cell simulations. 2) Application of the two-dimensional models to the magnetosphere - There has been some work on whether the two-dimensional models are applicable to the magnetosphere, but a more complete determination is necessary. In particular, is the extension to the two-dimensional model employed by Borovsky (2008) appropriate for the magnetosphere? If not, is there a simple way to extend the two-dimensional model to incorporate such effects? If not, can one develop a first principles understanding of the efficiency of three-dimensional magnetic reconnection in the dayside geometry? Can the effects of a large convective electric field be incorporated into the model? These studies can be carried out using idealized two- and three-dimensional magnetohydrodynamic, Hall-magnetohydrodynamic, hybrid, and particle-in-cell simulations, as well as using global magnetospheric simulations. The latter are readily available to the scientific community at large through NASA s Community Coordinated Modeling Center operated at Goddard Space Flight Center.

6 Theory and analysis of solar wind-magnetospheric coupling - 1) Development of increasingly accurate coupling functions - as the understanding of fundamental reconnection physics improves, can the coupling function based on local reconnection physics can be enhanced? If so, do the advanced coupling functions give better correlations with the data? 2) Empirical studies of coupling functions - the previous empirical work chose a small number of parameters and particular functional forms that were used in fits to the data. Can empirical models be improved using the forms resembling Borovsky s coupling function? If so, can the improved fits feed back to the theoretical side and provide clues about what physics should be incorporated into the coupling function? Satellite observations and data analysis - 1) Solar wind-magnetospheric coupling - Dayside reconnection is only the gateway to solar-wind/magnetosphere coupling: it magnetically connects the solar wind to the magnetosphere-ionosphere system. The coupling physics happens post-reconnection, with the reconnection rate controlling the amount of coupling. The physical processes that couple the MHD generator of the shocked solar wind to the magnetosphere and ionosphere have yet to be studied in earnest. Global MHD simulations, archival spacecraft and ground-based measurements, and new focused space missions should be marshaled upon this fundamental space-physics problem. 2) Properties of magnetic reconnection at the dayside magnetopause - Having more measured events of magnetic reconnection at the dayside is important for comparisons to simulations. In addition to present satellites such as Cluster and THEMIS/ ARTEMIS, unprecedented spatial resolution of reconnection sites will be available from the Magnetic Multi-Scale (MMS) mission. A goal of MMS is resolving electron physics as it passes through reconnection diffusion regions. Such information will be invaluable for verification of simulations and theories of reconnection. 3) Identifying physics left out of models - The present coupling function has a 75% correlation with the data. By looking at the physical circumstances of the data that does not fit well with the model, can one ascertain key physical processes that are present in the real system that are not modeled well by the equations? In summary, the topic of solar wind-magnetospheric coupling predictions is important with very high scientific value and has important implications for society at large. The activities are low risk and have a high level of technical readiness as much can be addressed using existing satellites and computational infrastructure.

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